This invention relates generally to geophysical exploration. More particularly, this invention is directed to a seismic exploration system including an adaptive seismometer group recorder having enhanced operating capabilities for acquiring, processing, and storing seismic signals.
Seismic exploration involves generating seismic waves at the surface of the earth by means of a seismic source. The seismic waves travel downwardly into the earth and are reflected and/or refracted due to differences in the elastic impedance at the interface of various subterranean formations. Detectors, called seismometers. or geophones, located along the surface of the earth, and/or in a borehole produce analog electric seismic signals in response to detected seismic wave reflections and/or refractions. The analog electric seismic signals from the seismometers, or geophones, can then be recorded. Alternatively, the analog electric seismic signals from the seismomters, or geophones, can be sampled and digitized prior to being recorded. The seismic data recorded in either manner are subsequently processed and analyzed to determine the nature and structure of the subterranean formations.
Various portable seismic exploration systems are known. One type of portable seismic exploration system employs cableless seismic recording systems developed for semismic prospecting by digitally recording seismic signals produced by seismometers or geophones without the need for multiconductor cables or alternate means such as radio or wire telemetry for transmitting seismic data to a central recording point. In particular, the cableless seismic recording system includes recorders placed near the seismometer, or geophone, locations and arranged for producing individual recordings in response to control signals transmitted from a control point over a communication point, preferably a radio communication link. A second type of portable seismic exploration system employes various telemetry systems, which merely relay the acquired seismic data by way of a radio communications link, or a fiber optic or electric cable, to a central recording location.
The forerunner of cableless seismic recording system disclosed by Montgomery U.S. Pat. No. 3,283,295 comprises a cableless seismic analog recording system wherein a radio receiver is associated with a recorder located at each seismometer, or geophone, location in the prospect area. The recorder is activated by control signals from a centrally located radio transmitter and thereafter records the analog seismic data. However, the cableless seismic analog recording system disclosed in Montgomery is limited to an analog recording of a seismic signal as a frequency modulated magnetic record which is inferior to digital recording, which has unexcelled accuracy, dynamic range, and freedom from noise interference. Additionally, Montgomery discloses that all remotely operated recorders are in operation for each recording. Reconfiguration of the array for each new recording involves repositioning the various recorders along the line of survey.
Broding, et al., U.S. Pat. No. 3,806,864, hereby incorporated by reference into this specification to form a part thereof, discloses a cableless seismic recording system which overcomes the two noted deficiencies of the cableless seismic analog recording system disclosed by Montgomery in that the recording produced is digital in format and out of a large array of recorders remotely deployed in one prospect area, only those recorders needed for producing a given set of recordings are selectably activated and caused to record seismic data. The remaining recorders remain essentially quiescent until there is a desire to produce a set of recordings for the prospect areas where they are situated. As disclosed in Broding, the seismic data are recorded on a magnetic tape cartridge. The recorded seismic data are filtered, amplified and digitized in accordance with a fixed menu provided by hard-wired circuitry of the recorder.
Many techniques for generating seismic waves are currently in use. An exploding dynamite charge is an example of a high energy seismic source which generates a sharp pulse of seismic energy. Vibrators, which generate a "chirp" signal of seismic energy and hammers are examples of low energy surface seismic sources. In the case of vibrators, the recorded seismic wave reflections and/or refractions are cross-correlated with a replica (called the pilot signal) of the original chirp signal in order to produce recordings similar to those which would have been produced with a high energy seismic source. This process is commonly referred to by its tradename, VIBROSEIS.
Unfortunately, the recorded seismic data always include some background noise in addition to the detected seismic waves reflected and/or refracted from the subsurface formation (referred to as a seismic signal). The ambient noise appears in many forms, such as atmospheric electromagnetic disturbances, wind, motor vehicle traffic in the vicinity of the prospect area, recorder electrical noise, etc. When a high energy seismic source is used, such as dynamite, the level of detected seismic signal is usually much greater than ambient noise.
The use of the cableless seismic recording system disclosed by Broding, et al., is most advantageous in instances when seismic data is generated by a high energy seismic source. However, when a low energy surface seismic source is used, such as a vibrator used in Vibroseis type seismic prospecting, the ambient noise can be at a level greater than the seismic signal. For that reason, Vibroseis-type seismic records are often produced from the repeated initiation of the low energy surface seismic source at about the same origination point, thereby producing a sequence of seismic data based on the seismic wave reflections and/or refractions that have traveled over essentially the same path and, therefore, have approximately the same travel times. Because the data storage capacity in commercially available, magnetic tape cartridges such as disclosed by Broding, et al., is limited, the capacity is not always adequate for recording every repetition individually, or accommodating the increased record length required when the low energy seismic source is used.
In order to lessen the impact of the limited data storage capacity of commercially available magnetic tape cartridges, seismic data generated by low energy seismic sources can be vertically stacked (summed or composited) prior to recording in order to economize tape usage. Weinstein, et al., U.S. Pat. No. 3,946,357 and Broding, U.S. Pat. No. 4,017,833 both disclose hard-wired digital circuitry in the recorder of a cableless seismic recording system for summing seismic data acquired by the recorder in accordance with a fixed menu.
Weinstein, et al., U.S. Pat. No. 3,946,357, discusses a recorder including an adder circuit which sums newly acquired seismic-trace data received from a shift register with previously accumulated seismic-trace data temporarily stored in random access memory between consecutive initiations of the seismic source, and the accumulated sum is later recorded on a magnetic tape cartridge. Broding U.S. Pat. No. 4,017,833 discloses a recorder including a plurality of recirculating dynamic shift registers connected in cascade for storing the accumulated sum between consecutive initiations of the seismic source in order to economize power consumption.
In spite of such developments, a need remains in the field of geophysical exploration for acquiring, processing and storing seismic data with a portable seismometer group recorder having means for electronically downloading operating programs providing a plurality of menus of recorder operating parameters into the portable seismometer group recorder. Electronically downloading operating programs into the seismometer group recorder provides an operator with a plurality of menus of recorder operating parameters to remotely select from such that the portable seismometer group recorder can be remotely, electronically reconfigured to acquire and process seismic data for various geological settings without the necessity or expense of making hard-wired modifications to or replacements of the circuitry of such portable seismometer group recorders. Additionally, a need exists to provide the portable seismometer group recorder with means responsive to coded signals, transmitted from a remote control unit, for selecting recorder operating parameters from a menu of recording operating parameters to electronically reconfigure the portable seismometer group recorder to process the acquired seismic data for different geological settings without having to physically retrieve the portable seismometer group recorder. The present invention comprises an adaptive seismometer group recorder and method of geophysical exploration directed to fulfilling such needs.